A quadrupole mass analyzer includes a quadrupole unit 40, an ion detector 46 placed at an exit of the quadrupole unit 40, and a driver circuit 50, as shown in FIG. 5. A quadrupole unit 40 is composed of four rod electrodes 41, 42, 43, 44 placed in parallel to and symmetrically around the z axis. The driver circuit 50 applies both a direct current (DC) voltage U and a high frequency alternate current (AC) voltage V.cos(.omega..multidot.t) simultaneously between a pair of electrodes 42 and 44 placed along the x axis and the other pair of electrodes 41 and 43 placed along the y axis. When ions are introduced into the center of an end of the quadrupole unit 40 while the DC/AC voltage is applied among the four rod electrodes 41, 42, 43, 44, only ions 45 having a specific mass can pass the quadrupole unit 40 and other ions 47 disperse before the ion detector 46. Since the specific mass of the ions that can pass through the quadrupole unit 40 is determined by the DC voltage U and the high frequency AC voltage V, the mass of ions 45 detected by the ion detector 46 can be scanned by changing the values of the voltages U and V with a certain correlation between them.
The four rod electrodes 41, 42, 43, 44 of the quadrupole unit 40 must be placed precisely symmetrically around the z axis to perform a correct mass analysis. Thus, conventionally, the four rod electrodes 41, 42, 43, 44 are securely held by a pair of ceramic holders 48 and 49 at both ends of the rod electrodes 41, 42, 43, 44, as shown in FIGS. 6 and 7, to prevent displacement of the rod electrodes within the quadrupole unit 40. Then the quadrupole unit 40 is inserted in a cylindrical case 52 as shown in FIG. 6, or placed on a base plate 53 as shown in FIG. 7, to correctly align with an ion entrance and with the ion detector 46 (not shown in FIGS. 6 and 7).
When the high frequency AC voltage is applied among the four rod electrodes 41, 42, 43, 44 as described above for a mass analysis, the ceramic holders 48 and 49 experience a high frequency alternating electromagnetic field, and heat is generated in the ceramic holders 48 and 49 due to the dielectric heating effect. As the temperature rises due to the dielectric heating, the ceramic holders 48 and 49 expand and sometimes distort, resulting in a displacement or a loss of symmetry of the rod electrodes 41, 42, 43, 44. In the conventional quadrupole mass analyzers as shown in FIGS. 6 or 7, the heat generated in the ceramic holders 48 and 49 hardly goes out because the area of contact between the cylindrical case 52 and the ceramic holders 48, 49 or between the base plate 53 and the holders 48, 49 is small and the cylindrical case 52 and the base plate 53 are made of stainless steel.